Electromagnetic lock

For a lock utilizing magnetism and a key, see magnetic keyed lock.

An electromagnetic lock, magnetic lock, or maglock is a locking device that consists of an electromagnet and an armature plate. By attaching the electromagnet to the door frame and the armature plate to the door, a current passing through the electromagnet attracts the armature plate, holding the door shut. Unlike an electric strike a magnetic lock has no interconnecting parts and is therefore not suitable for high security applications because it is possible to bypass the lock by disrupting the power supply. Nevertheless, the strength of today's magnetic locks compares well with that of conventional door locks and they cost less than conventional light bulbs to operate. Power supplies incorporating a trickle-charged lead-acid battery pack can be used to retain security for short-term power outages.

The electromagnetic lock was patented on May 2, 1989, by Arthur, Richard and David Geringer of Security Door Controls, an access control hardware manufacturing firm. The device outlined in their designs was the same in principle as the modern magnetic lock consisting of an electromagnet and an armature plate. The patent did not make any reference to the manufacturing methods of the electromagnet and detailed several variations on the design, including one that used a spring-loaded armature plate to bring the armature plate closer to the electromagnet. The patent expired on May 2, 2009.^[1]

This device was a shear magnetic lock as opposed to the original (and now ubiquitous) 'direct pull' electromagnetic lock and was an improvement on a 1984 patent cited in the same document.

1 Operation
2 Equations
3 Technical comparison
- 3.1 Advantages
- 3.2 Disadvantages
4 Installation
5 Holding force
6 Principle
7 Voltage
8 Application modes
9 References

Operation

The magnetic lock relies upon some of the basic concepts of electromagnetism. Essentially it consists of an electromagnet attracting a conductor with a force large enough to prevent the door from being opened. In a more detailed examination, the device makes use of the fact that a current through one or more loops of wire (known as a solenoid) produces a magnetic field. This works in free space, but if the solenoid is wrapped around a ferromagnetic core such as soft iron the effect of the field is greatly amplified. This is because the internal magnetic domains of the material align with each other to greatly enhance the magnetic flux density.

Equations

Using the Biot-Savart law, it can be shown that the magnetic flux density $B$ induced by a solenoid of effective length $l$ with a current $I$ through $N$ loops is given by the equation:

$B = \frac{\mu_0\mu_rIN}{l}$

The force $F$ between the electromagnet and the armature plate with surface area $S$ exposed to the electromagnet is given by the equation:

$F = \frac{B^2S}{2\mu_0}$

In both equations, $\mu_0$ represents the permeability of free space and $\mu_r$ the relative permeability of the core.

Although the actual performance of a magnetic lock may differ substantially due to various losses (such as flux leakage between the electromagnet and the conductor), the equations give a good insight into what is necessary to produce a strong magnetic lock. For example, the force of the lock is proportional to the square of the relative permeability of the magnetic core. Given the relative permeability of a material can vary from around 250 for cobalt to around 5000 for soft iron and 7000 for silicon-iron, the choice of magnetic core can therefore have an important impact upon the strength of a magnetic lock. Also relevant is the choice of current, number of loops and effective length of the electromagnet.^[2]

Technical comparison

Magnetic locks possess a number of advantages over conventional locks and electric strikes. For example, their durability and quick operation can make them valuable in a high-traffic office environment where electronic authentication is necessary.

Advantages

Easy to install: Magnetic locks are generally easier to install than other locks since there are no interconnecting parts.
Quick to operate: Magnetic locks unlock instantly when the power is cut, allowing for quick operation in comparison to other locks.
Sturdy: Magnetic locks may also suffer less damage from multiple blows than do conventional locks. If a magnetic lock is forced open with a crowbar, it will often do little or no damage to the door or lock.

Disadvantages

Requires continuous power: To remain locked, the magnetic lock requires a constant power source. The power drain of the lock is typically around 3 watts, far less than that of a conventional lightbulb (around 60 watts), but it may cause security concerns as the device will become unlocked if the power source is disrupted. By comparison, electric strikes can be designed to remain locked should the power source be disrupted. Nevertheless, this behaviour may actually be preferable in terms of fire safety.

Installation

The magnetic lock should always be installed on the inside (secure side) of the door. Installation is as simple as installing the header of the door frame for out-swinging doors or using a Z-bracket for in-swinging doors. It is important to make sure the armature plate and the electromagnet align as closely as possible to ensure efficient operation. Magnetic locks are almost always part of a complete electronic security system. Such a system may simply consist of an attached keycard reader or may be more complex, involving connection to a central computer that monitors the building's security. Whatever the choice of locking system, fire safety is an important consideration.^[3]

Holding force

A magnetic lock has a metal plate surrounded by a coil of wire that can be magnetized. The number of coils determines the holding force which characterizes the lock:

Micro Size: 300 lbf (1,300 N) holding force.
Mini Size: 600 lbf (2,700 N) holding force
Midi Size: 800 lbf (3,600 N) holding force
Standard Size: 1,200 lbf (5,300 N) holding force.

The standard size electromagnetic lock is used as a gate lock.

Principle

Most installations are surface mounts. For safety, magnetic lock, cables, and wires should be inserted in the door or be a flush mount. The magnetic lock is suitable for both in-swing and out-swing doors. Brackets (L bracket, LZ bracket, U bracket) are used to adjust the space between the door and lock. The principle behind the electromagnetic lock is to use electromagnetism to maintain the lock after energizing. The electromagnetism exploits the advantage of a solenoid. The holding force should be a collinear load and the lock and armature plate should be face-to-face in the correct position to achieve optimal operation.

Voltage

The power for an electromagnet lock is DC (Direct Current), around 6 W. The current is around 0.5 A when the power is 12 V DC. Generally, the specification of the electromagnet locks is dual voltages 12/24 V DC. Single voltage output can be required for 12 V DC or 24 V DC applications. The figure presents the relationship between voltage and holding force. When the current is fixed, voltage is proportional to power consumption.

Application modes

For safety purposes an electric lock has two modes:
Fail-Safe – to achieve human safety: The lock will be released when the power shuts down.
Fail-Secure – to achieve property safety: The lock remains closed when the power is shut down.

An electromagnet lock is used for Fail-Safe applications, and the lock should satisfy the specifications in fire regulations to be safe in emergency situations.

References

^ Geringer A. Geringer R. Geringer D. Electromagnetic Door Lock Device, U.S. Patent 4,826,223, May 2, 1989.
^ Sadiku, M. Elements of Electromagnetics (3rd edition), Oxford University Press, 2001 (ISBN 0-19-513477-X).
^ Performance Specifications, http://www.assa.co.uk/resources/Services/docs/PSG%20Word.rtf, ASSA Limited (Last updated 7th, May 2003)
^ The Complete Book of Locks and Locksmithing (4th edition), Bill Phillips, McGraw-Hill Inc. 1995.
http://bizarrelabs.com/mag.htm
Exploration of the Earth's Magnetosphere
http://www-spof.gsfc.nasa.gov/Education/Intro.html